1. Field of the Application
The invention relates to a leakage device for a turbomachine, more particularly for an axial turbomachine compressor, more particularly still, for an axial low-pressure compressor, the leakage device having, as an object, that of reducing, eliminating, or controlling the surging phenomenon which can occur in the area of the flow in the machine at certain operation speeds. The invention also relates to a wall equipped with at least one said leakage device, to a compressor equipped with said wall, as well as, to a process for reducing and suppressing a surging phenomenon in an axial compressor.
2. Description of Related Art
Surging is a fundamental phenomenon peculiar to dynamic compressors. The latter are characterized in that compression is achieved by power transfer in the fluid set in motion in rows of blades. Axial and centrifugal compressors belong to this category of turbomachines. Similar to the stalling phenomenon of an airplane wing which thus loses its lift, the fluid flow through the compressor can stall, which prevents the fluid from being “pushed”. Indeed, the compressor being connected to two circuits at different pressure, the high-pressure capacity of the discharge empties in the low-pressure capacity of the suction by a counter-current flow-rate in the compressor. Surging is the general aerodynamic instability of the compressor in its environment during operation, mostly at low flow-rate. When the discharge circuit has been sufficiently emptied in the suction, the compressor finds new operating conditions allowing the flow-rate to be restored in the right direction until a new instability cycle starts again. These large flow-rate fluctuations are called surges because of the nature of this relaxation oscillations phenomenon. The sudden flow-rate inversion is similar to a shock and its mechanical consequences can be disastrous, such as, for example, the breaking of the blades on the axial compressors.
Specific protection means must be installed on the machines to preserve their integrity and maintain their performance over time. These means are all the more complex because the largest range possible of the flow-rate is always desired for the operation of the machine in order to adapt the power in a permanent, economical, and flexible manner while maintaining the consumption as low as possible.
Numerous protection means against surging have been developed in the past. Most of them consist in generating a controlled leakage in the area of the wall of the compressor casing in the area of the end of a row of rotor blades.
The patent document GB 801,771 discloses a similar device consisting, essentially, of a series of movable plugs arranged on the perimeter of the compressor casing of an axial turbomachine, facing holes made in the casing. The movable plugs are connected in a sealed manner with a flexible membrane whose rear surface forms, with a wall element, a chamber for controlling the plugs. The front surface of the membrane forms a leakage chamber common to the different plugs and in connection with a valve enabling the leakage to be connected to the atmosphere or to a diffuser downstream of the compressor. This device has the advantage of being controllable, but the consequence of this measure is to require an efficient control device, which increases the complexity and the cost of the device. Furthermore, the construction itself of the movable plugs serving as leakage valves is rather complex and requires a lot of care, in the precision of execution as well as in terms of maintenance (replacement of the membranes, checking that the plugs correctly slide in their guides, etc).
The patent document GB 1,357,016 discloses a leakage device controlled according to a general principle similar to that of the previous document, namely, holes or openings made in the wall of the compressor casing, of an axial turbomachine being blocked up or kept free by the application of a membrane put under pressure by connection to the compressor. The membrane is displaced by means of a cylindrical wall surrounding the cylindrical casing of the compressor perpendicularly to the openings. The membrane is fixed to this cylindrical wall so that the displacement of this wall in parallel to the casing wall displaces the membrane by rolling onto itself, enabling the openings to be opened or blocked up. Compared to the teachings of the previous document, this device has the advantage of being able to control the leakage but requires complex and costly control means.
The patent document GB 2,158,879 A discloses an active leakage device, that is, controlled. It essentially consists of a valve put on a discharge circuit in the vicinity of a duct connecting an opening made in the casing of an axial turbomachine compressor to the discharge circuit. The valve is controlled by an actuator which is also controlled. Compared to the teachings of the previous documents, this device has the advantage of making it possible to control the leakage but requires complex and costly control means.
The active leakage devices such as described hereinabove generally consist in withdrawing a variable flow-rate of air according to a defined program. A set of adjustments must then be defined and controlled by the FADEC (Full Authority Digital Engine Control). The FADEC is a system which lies in a calculator and control and command peripherals being interfaced between the cockpit and the aircraft engine. These adjustments being pre-defined, the functioning is not optimal, in particular during the transients.
The patent document EP 1,898,067 A discloses a passive leakage device for a jet engine compressor. The device creates a permanent leakage with an upstream recirculation, which results in losses and a non-optimal use of the compressor. The surge adjustment requirement thus causes a permanent degradation of the performances of the compressor, due to the presence of a leakage even in the absence of any penalizing phenomenon.
Although great strides have been made in the area of axial compressors, many shortcomings remain.